CD40 is a 48 kDa type I integral membrane glycoprotein and a member of the tumor necrosis factor (TNF) receptor superfamily. CD40 is expressed on a variety of cell types including normal and neoplastic B cells, interdigitating cells, carcinomas, epithelial cells (e.g. keratinocytes), fibroblasts (e.g. synoviocytes) and platelets. It is also present on monocytes, macrophages, some endothelial cells, and follicular dendritic cells. CD40 is expressed early in B cell ontogeny, appearing on B cell precursors subsequent to the appearance of CD10 and CD19, but prior to expression of CD21, CD23, CD24, and appearance of surface immunoglobulin M (sIgM) (Uckun et al., 1990, Blood 15:2449). CD40 has also been detected on tonsil and bone marrow-derived plasma cells (Pellat-Decounynck et al., 1994, Blood 84:2597).
The ligand of CD40 is CD40L (also referred to as CD154, gp39, and TRAP), a TNF superfamily member. CD40L is a transmembrane protein expressed predominantly on activated CD4+ T cells and a small subset of CD8+ T cells (Reviewed by (Van Kooten C. and Banchereau, 2000).
The interaction of CD40 with CD40L induces both humoral and cell-mediated immune responses. CD40 regulates this ligand-receptor pair to activate B cells and other antigen-presenting cells (APC) including dendritic cells (DCs) (Reviewed by (Toubi and Shoenfeld, 2004); (Kiener, et al., 1995). The function of CD40 on B cells has been studied extensively. Activation of CD40 on B cells induces proliferation, differentiation into antibody secreting cells and isotype switching in germinal centers of secondary lymphoid organs. In vitro studies have shown direct effects of CD40 activation on cytokine production (IL-6, IL-10, TNF-α, LT-α), expression of adhesion molecules and costimulatory receptors (ICAM, CD23, CD80 and CD86), and increased expression of MHC class I, MHC class II, and TAP transporter by B lymphocytes (Liu, et al., 1996). For most of these processes, CD40 acts in concert with either cytokines or other receptor-ligand interactions.
CD40 signaling on monocytes and DCs results in enhanced survival as well as secretion of cytokines (IL-1, IL-6, IL-8, IL-10, IL-12, TNF-α and MIP-1α). CD40 ligation on these APCs also leads to the up-regulation of costimulatory molecules such as (ICAM-1, LFA-3, CD80, and CD86). Activation of CD40 receptors is one of the critical signals that allow the full maturation of DC into efficient APCs driving T cell activation (Banchereau and Steinman, 1998) (Van Kooten C. and Banchereau, 2000).
Recent studies in mouse models showed that CD40 signaling on dendritic cells also plays an important role in the generation of TH17 cells which are considered as mediators of autoimmunity in diseases such as arthritis and multiple sclerosis (lezzi, et al., 2009) (Perona-Wright, et al., 2009).
The availability of CD40 and CD40L knock-out mice as well as agonistic and antagonistic anti-mouse antibodies offered the possibility to study the role of CD40-CD40L interactions in several disease models. Administration of blocking anti-CD40L has been demonstrated to be beneficial in several models of autoimmunity including spontaneous diseases like lupus nephritis in SNF1 mice or diabetes in NOD mice or in experimentally induced forms of disease like collagen-induced arthritis (CIA) or experimental autoimmune encephalomyelitis (EAE) (Toubi and Shoenfeld, 2004). CIA in mice was inhibited by an anti-CD40L mAb which blocked the development of joint inflammation, serum antibody titers to collagen, the infiltration of inflammatory cells into the subsynovial tissue in addition to the erosion of cartilage and bone (Durie, et al., 1993). Both for lupus nephritis and EAE, it was demonstrated that anti-CD40L could also alleviate ongoing disease, confirming the role of CD40-CD40L in the effector phase of the disease (Kalled, et al., 1998); (Howard, et al., 1999).
The role for CD40-CD40L interactions in the development of EAE was also studied in CD40L-deficient mice that carried a transgenic T cell receptor specific for myelin basic protein. These mice failed to develop EAE after priming with antigen, and CD4+ T cells remained quiescent and produced no INF-γ (Grewal, et al., 1996).
Furthermore, inhibitory antibodies directed against CD40 showed beneficial effects in inflammatory disease models such as EAE. Lamann and colleagues demonstrated that the antagonistic mouse anti-human CD40 mAb mu5D12 and a chimeric version of this mAb effectively prevented clinical expression of chronic demyelinating EAE in outbred marmoset monkeys (Laman, et al., 2002); (Boon, et al., 2001). A follow-up study showed that therapeutic treatment with the chimeric anti-human CD40 antibody reduces MRI-detectable inflammation and delays enlargement of pre-existing brain lesions in the marmoset EAE model (Hart, et al., 2005).
Anti-CD40 antibodies with agonistic activity were tested in mouse models of arthritis with some conflicting results. As expected for an immunostimulatory agent, the agonistic anti-mouse CD40 mAb FGK45 was shown to exacerbate disease in the DBA/1 mouse model of CIA (Tellander, et al., 2000). However, in another chronic CIA model FGK45, and another agonistic anti-mouse CD40 mAb, 3/23, both exhibited positive therapeutic effects (Mauri, et al., 2000). It was postulated by this group that the agonistic antibodies in this therapeutic treatment regimen have a beneficial effect by inducing immune deviation towards a Th2 response with decreased levels of IFN-γ and increased levels of IL-4 and IL-10 (Mauri, et al., 2000).
The prevention of transplant rejection by blocking CD40/CD154 interactions has also been documented. The use of ch5D12, a chimeric anti-CD40 antagonist, in renal allograft studies in rhesus monkeys indicates that antagonism of CD40 is sufficient for disease modification and lengthening mean survival times past 100 days. When ch5D12 was combined with an anti-CD86 antibody and given only at the initiation of the allograft studies followed by prolonged treatment with cyclosporine, mean survival times greater than 4 years were achieved, indicating this combination can potentially induce tolerance (Haanstra, et al., 2005).
Thus, there are ample preclinical studies that provide evidence for the crucial role of the CD40-CD40L dyad in driving an efficient T cell-dependent immune response. Blocking of CD40 signaling is therefore recognized as a suitable and needed therapeutic strategy to suppress a pathogenic autoimmune response in diseases such as RA, multiple sclerosis or psoriasis. However, to date, there are no CD40 antibodies that have been approved for therapeutic intervention of such disorders due to the findings that anti-CD40 antibodies previously in development were shown to have significant side effects. Thus, there remains a significant need for therapeutic agents that can be used to intervene in the action of the CD40-CD40L and block CD40 signaling. This need could be addressed by new humanized anti-CD40 antibodies that specifically bind CD40 and which show the antigen binding specificity, affinity, and pharmacokinetic and pharmacodynamic properties that allow use thereof in therapeutic intervention of CD40 based disorders.